Oh, I wholeheartedly agree. My defensive handguns, I have at least 15 rounds on me. More often 25. But this is the one thing I don't like about single-stack magazines. But dangit, I like my 1911...
I'm honestly thinking that my short Mosin-Nagant with several stripper clips may do ok for a truck gun. Slow to reload, but definitely not underpowered, and to be blunt, that side folding bayonet is there for a reason.
what I also do is keep several additional magazines in the center console for my pistol so that if I am having to use that without getting a chance to employ the favored longgun I've got plenty of ammunition on tap.
I figure that if I am out of my vehicle and in a store then the threat is likely to be handled by the two spare mags I carry on body but if I am being assaulted in/at my vehicle by a mob/riot/whatever then I've got plenty of rounds on hand to dole out as early Christmas presents...
And should I run out of the ammunition in the vehicle then I've got the vehicle itself, ax, kuhkri, tomahawk, fire extinguisher, crowbar (I LOVE me a good crowbar), etc to settle accounts personally.